Python Interview Question - Beginner to Advance

Input = "AAAABBBCCDAABBB"
Output= A4B3C2D1A2B3

# define a function() that takes string input and return null if string is null and returns 1st occurrences of character followed by number of occurrences and so on. 

# This function takes a string and returns processed string output. 
def str_skimmer(input_string):
    # Return null string if input is null. 
    if not input_string:
        return ""
    # Store values for 2 parameters - "Reference Character and "Count"
    prev_char=input_string[0]
    op=[]
    count=1

    for i in input_string[1:]:
        if (i==prev_char):
            count += 1 
            print(i, count, op)
        else: 
            op.append(prev_char+str(count))
            prev_char=i
            count=1
    op.append(prev_char+str(count))  
    print(op)

    return ''.join(op)
#O: A4B3C2D1A2B3

ip="AAAABBBCCDAABBB"
op= str_skimmer(ip)
print(op)

Input = "TBD"
Output= TBD